An auger type ice making machine has a refrigeration casing that is a vertically elongated cylindrical member. A cooling pipe that constitutes an evaporator of a refrigeration circuit is wrapped around an outer circumferential surface of the refrigeration casing, and an auger having a helical blade is provided in an inner portion thereof. Ice making water is supplied to the inner portion of the refrigeration casing. Ice that grows on an inner circumferential surface of the refrigeration casing is scraped off by rotation of the helical blade, becoming flake ice, and is conveyed upward by helical action. A pressing head is disposed in an upper portion of the refrigeration casing in order to form the ice into a predetermined shape and predetermined hardness.
However, the refrigeration casing may become overcooled when, for some reason or another, there is an ice blockage in an internal portion of the pressing head, when there is an insufficient supply of ice making water, when an abnormality occurs in the refrigeration circuit, or the like. Ice making water in the inner portion of the refrigeration casing completely freezes if the ice making machine is driven in this state. Not only is there an excessive load applied to the auger, a geared motor that drives the auger, the refrigeration casing, an upper bearing, and a seal that partitions the geared motor and the ice making water, there is also a possibility that damage may occur to the auger, the geared motor, the refrigeration casing, the upper bearing, the seal, and the like.
There is conventionally a method in which an overload relay is used as a protecting device for protecting a geared motor against this problem by stopping the geared motor when the overload relay detects that a load applied to the geared motor has exceeded a predetermined value. As shown in FIG. 14a, this is a method in which the overlay relay operates when the geared motor is locked and a lock current higher than a motor current during normal operation flows for a fixed period of time.
If so-called hunting occurs, however, where forward rotation and reverse rotation of the auger are repeated, beginning with the forward rotating auger being unable to scrape off the ice for some reason or another, the auger then rotating in reverse due to the impact, and in addition, the auger again rotating forward due to a collision with the ice, the motor current will fluctuate as shown in FIG. 14b. The current value repeatedly moves back and forth between the lock current and a value in the vicinity of the electric current during normal operation. Consequently, the overload relay does not operate, and the geared motor cannot be protected.
A protecting device is proposed in JP 4-24625 B in which current flowing in a geared motor is converted to voltage, and operation of the geared motor is stopped when the converted voltage becomes larger than a predetermined value. According to this protecting device, operation of the geared motor is stopped when the current flowing in the geared motor increases and the converted voltage becomes larger than the predetermined value, even for an instant, during ice making operations. It therefore becomes possible to stop the geared motor even during hunting.
However, the motor current flowing in the geared motor also fluctuates due to the value of an input voltage that is applied to the geared motor. Accordingly, there is a fear that an overload state may be judged to have occurred whenever the motor current fluctuates, even during normal ice making operation, causing operation of the geared motor to be stopped.